U.S. Pat. No. 4,659,016 to Faulkner, assigned to the assignee of the present invention, describes a dispenser for hot-melt adhesive. Adhesive from a heated hose enters a flow passageway in a solenoid assembly and is selectively channeled from the flow passageway to a number of nozzles on a dispensing bar. An armature in the flow passageway of the solenoid is biased into a blocking position to prevent fluid communication between the heated hose and the nozzles. Energizing the coil of the solenoid creates a magnetic field which attracts the armature. The magnetic force overcomes the bias on the armature and opens the path to release of hot-melt material from the nozzles. Subsequent de-energization of the coil releases the armature to again block passage of the viscous material.
The dispenser described in the above-mentioned patent to Faulkner is illustrated as used in the sealing of large cardboard boxes on an assembly line. In the sealing of cardboard boxes on packaging assembly lines, the timing tolerances for valving the adhesive on or off need not be exacting. Delays of 0.05 seconds are exceptable. However, in other applications timing may be critical. For example, it may be desirable to apply hot-melt adhesive in small, discreet quantities dispensed in rapid succession to form a row of dots on a work surface. The dot pattern is possible only if adhesive flow from dispenser nozzles is more responsive to energizing and de-energizing the coil of solenoid. That is, opening time and closing time of the solenoid is critical.
As noted above, in the solenoid assembly of Faulkner, the armature is biased so that the downstream end of the armature blocks flow to the dispenser nozzles. Upon energization of the solenoid coil, the generated magnetic field must overcome the force of a spring which biases the armature in the closed position and must overcome the resistance of the viscous adhesive material since the adhesive must be displaced to permit movement of the armature. Thus, maximization of magnetic force is a key factor in the opening time of the solenoid. Migration of adhesive also plays an important factor in determining the closing time of the solenoid. In the open position, the upstream end of the armature abuts a downstream end of a pole piece of the solenoid. Upon deenergization of the solenoid coil, the spring biasing of the armature into the closed position must overcome the force created by the abutment of the ends of the armature and the pole piece. The migration of adhesive between the two ends is a slow process since adhesive is an extremely viscous material and since the pumping pressure for adhesive application is often 500-600 psi.
It is an object of the present invention to increase the responsiveness of an electromagnetic valving assembly for viscous fluid material such as hot-melt adhesive.